Powder coating composition and method

ABSTRACT

A laminar pigment which has an aluminum- or an aluminum-cerium oxide coating combined with a hydrolyzed silane coupling agent treated surface is used as a pigment for a powder coating composition. By combining the pigment with a powdery film-forming polymer, a film-forming composition which can be applied by electrostatic coating is produced.

BACKGROUND OF THE INVENTION

The electrostatic coating process for the application of color coatingson various substrates is well known. The composition which is utilizedin this process is an electrically chargeable powder mixture of afilm-forming polymer and a pigment (or pigments) having the desiredcolor. Unfortunately, preparation of such powder coating compositionsfor application by a powder spray applicator has presented a problem inthe past in connection with platy pigments.

Platy pigments of a metallic material rely on their laminar structurefor maximum appearance effect. Such plates include, for instance,metallic flakes such as aluminum, bronze and stainless steel plates aswell as natural or synthetic pearlescent pigments exemplified by, e.g.,natural pearlescence or a metal oxide- coated substrate such as titaniumdioxide-coated mica, iron oxide-coated mica, titanium dioxide-coatedglass, iron oxide-coated glass and iron-coated aluminum flakes. Thelaminar structure of such metallic or pearlescent pigments is destroyedduring the extrusion or grinding processes which are used in themanufacture of powder coating compositions and as a result, the coatingappearance achieved exhibits a reduced luster effect.

The industry has attempted to avoid the foregoing problem by dryblending the pigments and polymer powder, i.e., the polymer carrier ismechanically blended with the pigment. Unfortunately, the pigment andthe powder particles usually develop a different charge magnitude whichresults in a color shift when the powder coating composition iselectrostatically sprayed on a substrate. In addition, the pigments tendto separate from the powder in that volume of the powder spraycomposition which does not attach to the substrate and is thenrecovered, which makes that recovered material difficult to reuse.

To overcome the problems encountered as a result of the dry blendingprocess, a blending process in which the powder base and the pigmentwere mixed and then heated to a temperature sufficient to soften thesurface of the powder particles so that the pigment could bind to thesurface of such particles was developed, as described in U.S. Pat.5,187,220. While this process works well for corona electrostaticcharging and alleviates the problems with reusing the powder overspray,it also leaves two distinctly different surfaces in the final mixture.The procedure also does not work well for tribo electrostatic charging.

A significant advance in the art is described in U.S. Pat. No.5,824,144, the disclosure of which is hereby incorporated by reference.As there described, the metal containing platelet pigment is providedwith a viscous surface layer of polymer or other sticky liquid material.When that treated pigment is blended with powder coating composition,the powder attaches to the surface of the pigment thereby minimizingcolor separation. The pigment is also encapsulated by the powder tothereby present a single surface. A better charge of those pigmentparticles which do not become attached to the powder materials is alsorealized.

A significant problem in connection with the use of metal-containingplatelet pigments in a powder coating composition is that theconcentration of the pigment which can be incorporated is limited. Whilepigment amounts of up to about 10% or more can be utilized in singleapplication environments or in the laboratory, the concentration islimited to about 3% when operating on a larger scale as is requiredcommercially. At levels higher than about 3%, numerous problems arise.These include spray gun clogging, blotchy panel appearance and colorseparation.

It has now been discovered that a treatment which had previously beendeveloped for pearlescent pigments intended for exterior use to provideimproved humidity, resistance and overall weather ability and which wasused in liquid coating systems such as a solvent or water borneautomotive paint systems, for example, pigmented base coat and clear topcoat, provides superior properties in the context of powder coatings.The exterior pigment treatment surprisingly improves the applicationproperties of the pigment when incorporated into powder coatings. Suchimprovements include less build up of the pigment at the electrostaticgun tip, improved transfer through feed lines, improved transferefficiency of the pigment and a more uniform coating appearance.

SUMMARY OF THE INVENTION

The present invention relates to an improved pearlescent pigment powdercoating composition and powder coating method. More particularly, theinvention relates to a powder coating composition comprising a mixtureof a particulate resin carrier and a particulate pigment in which thepigment is a laminar metal-containing pigment having a first coating ofa hydrated aluminum oxide or hydrated cerium and aluminum oxides and acoating of a hydrolyzed silane coupling agent on or intermingled withthe first coating. Use of this powder coating composition provides animproved powder coating application method.

DESCRIPTION OF THE INVENTION

The powder coating composition of the present invention is theconventional powder coating composition for electrostatic application asa result of dry blending in which the pigment has been treated asdescribed below. Thus any known particulate resin carrier usedheretofore can also be used in the composition and process of thepresent invention. As a result of the improved pigment utilized in thepresent invention, the pigment concentration can be typically up toabout 8% of the total powder. Typically, depending on the effectdesired, the pigment will constitute about 1 to 8% of the composition.

The pigment of the present invention is laminar, that is, it is aplatelet-like pigment. The platelets can be made of a reflective metalsuch as aluminum, bronze or stainless steel in the form of flakes oralso can be any natural or synthetic pearlescent pigment. Naturalpearlescent pigments include natural pearlescence and the syntheticpigments include metal oxide-coated mica pigments, metal oxide-coatedglass flakes, iron-coated aluminum flakes and reduced titanium-coatedmicas. On a commercial basis, the titanium dioxide and iron oxide micasare encountered most often. Nevertheless, any laminar pigment can betreated in accordance with the present invention.

Coating of a metal oxide-coated mica pigment with a hydrous aluminumoxide per se is known. It is described, for example, in U.S. Pat.No.5,091,011, the disclosure of which is incorporated herein byreference. Briefly, the pigment is dispersed by stirring in water andthen an aluminum compound such as aluminum chloride, aluminum sulfate oraluminum potassium sulfate, and a neutralizing agent such as sodiumhydroxide, potassium hydroxide ammonia or urea, are added simultaneouslyas aqueous solutions. The resulting hydrolysis causes the hydrous oxideto deposit on the substrate. As described, the aluminum compound must beadded slowly enough to permit the formation of a smooth, continuouslayer on the platelets and the rate should fall within the range ofabout 0.03 to 0.1 mg A1 per minute per gram of pigment, preferably about0.005 to 0.07 mg A1/min/g pigment. A quantity of aluminum compoundsolution is used so as to produce a hydrous aluminum oxide coatingcontaining about 0.05 to 1.2% aluminum, preferably about 0.1 to 0.8%aluminum, based on the total weight of the pigment. Pigments in whichthe concentration of aluminum is about 1.2% are less effective instabilization than lower concentrations. After deposition of thecoating, the product can be filtered, washed with water and dried at anyconvenient temperature. Use of a temperature which is high enough tocalcine the hydrous aluminum oxide should be avoided.

The coating of the pigments with a coating consisting essentially of acombination of hydrated cerium and aluminum oxides is also per se known.It is described, for instance, in U.S. Pat. No. 5,423,912, thedisclosure of which is incorporated herein by reference. Briefly, thepigment is dispersed in a liquid from which the cerium and aluminum canbe readily precipitated onto the surface of the pigment. Thisconveniently, and preferably, is an aqueous dispersion. The solidpigment in the dispersion generally comprises about 5 to 30%, preferablyabout 10 to 20%, and the cerium and aluminum are each added to thedispersion in the form of a salt which is soluble in a liquid medium.While other salts can be used, the nitrate salts are preferred. It isalso preferred to deposit about 0.1-1.5% cerium hydroxide, morepreferably 0.2-0.6%, calculated as weight percent cerium and about0.1-1%, more preferably 0.2-0.6%, aluminum hydroxide, calculated asweight percent aluminum, based on the weight of the pigment. The saltscan be added to the slurry individually in either order and precipitatedor preferably, added simultaneously and precipitated. Precipitation iscontrolled by raising the pH to a value greater than about 5, preferablyto a value of about 5.5-7.5. After completion of the precipitation step,the treated product is separated from the dispersion by any convenientmeans such as, for instance, filtration, centrifugation or settling,washed and dried.

The aluminum- or aluminum-cerium-treated pigment of this invention isadditionally treated with a hydrolyzed silane coupling agent or amixture of such agents. These, as known, are compounds which act as aninterface between an organic material and an inorganic material toenhance the affinity between the two. Thus, the silane coupling agentsgenerally have both an organo functional group and a silicon functionalgroup bonded either directly or indirectly to silicon. The siliconfunctional groups are generally alkoxy groups and preferably C₁₋₄ alkoxygroups.

Examples of silane coupling agents which can be used in the presentinvention are gamma-(2-aminoethyl)aminopropyltrimethoxysilane,aminopropyl trimethoxysilane, gamma-aminopropyltriethoxysilane,gamma-(2-aminoethyl)aminopropylmethyldimethoxysilane,gamma-methacyryloxypropylmethyltrimethoxysilane,gamma-metacyryloxypropyltrimethoxysilane, gamma-glycidoxypropyltrimethoxysilane, gamma-mercaptopropyltrimethoxysilane,vinyltriacetoxysilane, gamma-chloropropyltrimethoxy silane,vinyltrimethoxysilane, trimethylchlorosilane,gamma-isocyanatopropyltriethoxysilane and the like.

The silane coupling agent should be selected so that it is suitable forany organic material which will be combined with the pigment in use.

The pigment is treated with the silane coupling agent by dry or wetmixing. For instance, an aqueous solution of the agent in water or amixture of water and an organic solvent can be added to an aqueousslurry of the pigment. The silane is preferably prehydrolyzed such as,for instance, by stirring the coupling agent in water for a suitableperiod of time. It is also possible to effect hydrolyzation at the timeof mixing. In general, about 0.1 to 10 wt %, preferably about 0.25 to 5wt %, of the silane coupling agent is used based on 100 parts by weightof pigment being treated. The coupling agent and pigment are combinedfor a period of time sufficient to allow reaction to occur, which maylast from a few minutes to several hours or more, preferably about 3 to24 hours. Thereafter the treated pigment can be recovered in theconventional fashion such as by filtration, centrifugation and the like,and dried. It is also possible to combine, if desired, the couplingagent treatment with the aluminum/cerium treatment.

In order to further illustrate the present invention, variousnon-limiting examples are set forth below. In these examples, asthroughout the specification and claims, all parts and percentages areby weight and all temperatures in ° C., unless otherwise indicated.

EXAMPLE 1

A blue-reflecting TiO₂-coated mica pigment containing 53% TiO₂ and 1%SnO₂, with platelets primarily 5 to 50 ,μm in length, was used as thesubstrate. Aliquots of the pigment (250 g) were dispersed in 3 liters ofdistilled water and heated with stirring at 60° C. The pH was adjustedto 5.5 with hydrochloric acid and then a solution of 2.91% AlCl₃.6H₂O(3.3 mg Al/ml) was added at a rate of 4 ml/min for 57 minutes.Simultaneously, a 3.5% caustic solution was added so as to maintain thepH at 5.5. After stirring for 15 minutes, the product was filtered,washed with distilled water and dried at 110° C. The hydrous aluminumoxide contained 0.3% aluminum based on the total weight of the treatedpigment.

A charge of 400 g of the aluminum surface treated titaniumdioxide-coated mica was placed in a V-shaped tumble blender equippedwith an intensifier bar. A hydrolyzed silane coupling agent had beenprepared by stirring 10 g of gamma-glycidoxypropyltrimethoxysilane and22 g of deionized water for 1 hour. The hydrolyzed silane is in anamount of 2.44 g was atomized onto the pigment as it was being tumbledin the blender and intensely mixed for about 20 minutes in order toevenly distribute the silane on the pigment. Thereafter the combinationwas allowed to stand for two hours in order to permit the reactionbetween the coupling agent and the pigment to reach completion.

EXAMPLE 2

The procedure of Example 1 was followed except that the pigment was ared-reflecting ferric oxide-coated mica (40% Fe₂O₃) containing plateletsprimarily 5 to 40 μm in length. The aluminum chloride solution was addedfor 114 minutes and the product, after filtering, washing and drying,contained a hydrous oxide having 0.6% aluminum.

EXAMPLE 3

A 2% solution of (3-glycidoxypropyl)trimethoxy silane in water, adjustedto a pH of 4 with acetic acid, was allowed to stir for 60 minutes.Enough aluminum treated titanium dioxide-coated mica of Example 1 wasadded to the solution to obtain a slurry concentration of 20%, all thewhile stirring the slurry. After about 2.5 hours, the pigment wasrecovered from the aqueous slurry by filtration and dried at about 80°C., for 16 hours.

EXAMPLE 4

The procedure of Example 3 was repeated except that the silane employedwas β-(3,4-epoxycyclohexyl) ethyltrimethoxysilane.

EXAMPLE 5

The procedure of Example 3 was repeated except that the silane wasgamma-methacryloxypropyltrimethoxysilane.

EXAMPLE 6

The procedure of Example 3 was repeated except that the silane was amixture of gamma-glycidyloxypropyl trimethoxysilane andoctyltriethoxysilane.

EXAMPLE 7

Eight parts of a titanium dioxide-coated mica pearlescent pigment whichhave been treated with cerium hydroxide and silane following theprocedure of Example 1 and having a pearl appearance was dry blendedwith 100 parts of a black polyester powder (RAL9005 supplied by H. B.Fuller under the commercial code P3M5855). The polyester powder andpigment were mixed and shaken for 5 minutes, sifted first through a 30mesh screen and then through a 60 mesh screen and the collected materialwas shaken for an additional 5 minutes. The blend was thenelectrostatically applied to aluminum panels using a corona spray gunand the panels were cured for 10 minutes in an electric oven. Theresultant panels presented a smooth, uniform appearance and build up onthe spray gun nozzle tip was not apparent.

For comparison purposes, the foregoing procedure was repeated exceptthat the pigment was a pearl appearing chrome treated exterior gradepearlescent pigment which was commercially available. There wasconsiderably more pigment build up on the spray gun tip in this instanceand the coating obtained was less uniform in appearance and darker incolor, which indicated that a lower percentage of the pigment waspresent on the panel.

EXAMPLE 8

In another experiment designed to measure the transfer efficiency, thetreatments described in Example 7 were repeated. Before the coatedpanels were baked, the coating powder was scraped off the panels andanalyzed to determine the transfer efficiency and the mean diameter ofthe particles before and after spraying. The amount of pigment in thesamples was determined by ashing the samples and quantitatively weighingthe residue. If all of the pigment had been transferred to the panel,the amount of pigment in the ash would be the same as the amount in thedry coating mixture.

The results are set forth in the following table in which “Base” refersto the RAL9005 without any pigment, “Ce/Si” refers to the dry blendpowder using the pigment of the invention, “Cr Dry Blend” refers to thedry blend powder using the comparative chromium treated pigment and “CrBonded” refers to the product of the blending process the powder baseand pigment, and heating to the softening point resulting in the bondingof the pigment to the powder surface as described in the aforementionedU.S. Pat. No. 5,187,220 applied to the chromium treated pigment.

TABLE 1 % Mean Particle Pigment in Ash Size (μ) Before After TransferBefore After Composition Spray Spray Efficiency Spray Spray Base — — —42.3 44.1 Ce/Si 4.88 4.39 90 29.4 28.9 Cr Dry Blend 4.02 2.72 68 26.630.7 Cr Bonded 3.83 3.29 86 26.6 30.2

EXAMPLE 9

Example 7 was repeated except that the powder and pigment were mixed andthen only shaken for 5 minutes prior to spraying. The use of thepearlescent pigment of the invention appeared to impart a fluidity tothe finished powder which aided in the fluidization in the hopper andtransfer tubes of the coating equipment. The powder was observed to flowfreely without leaving residual material collecting in the transfertube. During spraying, the pigment did not appear to separate from thepowder and there was no residual pearl pigment on the deflector. Usingother pearl pigments which had not been treated according to theinvention, it was noted the pearl pigment collected on the deflectorperiodically clumps or transferred to the panel being coated yieldingmottling and defects.

EXAMPLE 10

Example 7 was repeated except that an iron oxide-coated mica pigment wasemployed. Improvements in smoothness are noted.

Various changes and modifications can be made in the process andproducts of the present invention without departing from the spirit andscope thereof. The various embodiments which have been described hereinwere for the purpose illustrated in the invention were not intended tolimit it.

What is claimed is:
 1. A powder coating composition comprising a mixtureof a particulate resin carrier and particulate pigment in which thepigment is a laminar metal-containing pigment having a first coatingthereon of hydrated aluminum oxide or a combination of hydrated ceriumand aluminum oxides, and a coating of hydrolyzed silane coupling agenton or intermingled with the first coating.
 2. A powder coatingcomposition according to claim 1 in which the silane coupling agentcomprises a non-amino silane coupling agent.
 3. A powder coatingcomposition according to claim 2 in which the non-amino silane couplingagent contains an alkoxy moiety.
 4. A powder coating compositionaccording to claim 3 in which the pigment is a titanium dioxidecoated-mica or an iron oxide-coated mica.
 5. A powder coatingcomposition according to claim 1 in which the silane coupling agent is atrimethoxysilane coupling agent.
 6. A powder coating compositionaccording to claim 1 in which said silane coupling agent is acombination of a non-amino silane coupling agent and an amino silanecoupling agent.
 7. A powder coating composition according to claim 1 inwhich the silane coupling agent comprises an alkyl alkoxy silane.
 8. Apowder coating composition according to claim 1 in which the pigment isa titanium dioxide coated-mica.
 9. A powder coating compositionaccording to claim 1 in which the pigment is an iron oxide-coated mica.10. A method of forming the powder coating composition of claim 1 whichcomprises dry blending a particulate resin carrier and particulatepigment, wherein the pigment is a laminar metal-containing pigmenthaving a first coating thereon of hydrated aluminum oxide or acombination of hydrated cerium and aluminum oxides, and a coating ofhydrolyzed silane coupling agent on or intermingled with the firstcoating.
 11. A method according to claim 10 in which the silane couplingagent comprises a non-amino silane coupling agent.
 12. A methodaccording to claim 11 in which the non-amino silane coupling agentcontains an alkoxy moiety.
 13. A method according to claim 10 in whichthe pigment is a titanium dioxide coated-mica.
 14. A method according toclaim 10 in which the pigment is an iron oxide-coated mica.